The cytotoxicity of gold nanoparticles(AuNPs) and the effects on cell growth have been reported trememdously, however the experiment results are often contradicted. The studies on the underlying mechanism are even rarely reported.A core problem in these researches is whether the AuNPs are toxic or benefit to the cell growth. It is understandable that the reactions of AuNPs in human body is much more complicated than those observed in vitro. Multiple components are involved in the human body, causing different immune responses and metabolism outputs. Nevertheless, microscopic investigation outside the human body still is a very important method to provide evidences for the mechanism of the role of AuNPs playing in the body.This review mainly introduces the results of the cytotoxicity of AuNPs and the effects on cell growth in vitro in terms of the size, surface charge and component, and the aggregation of AuNPs. The possible machanism involving in the nanoparticle effect has been discussed.

Contents
1 Introduction
2 Influence of size and surface property of gold nanoparticles (AuNPs) on the cytotoxicity
2.1 Influence of AuNPs size on the cytotoxicity
2.2 Influence of surface charge and protective layer of AuNPs on the cytotoxicity
2.3 Endocytosis
2.4 Influence of cell growth medium on the AuNPs aggregation state
3 The cytotoxicity assessment system and the mechanism of cytotoxicity of AuNPs
3.1 Assessment methods for cytotoxicity
3.2 Kinetic study of cytotoxicity process: Influence of AuNPs concentration and interaction time
3.3 Possible mechanism of cytotoxicity caused by AuNPs
4 Influence on cell growth caused by AuNPs
5 Conclusion